Method of fabricating group-III nitride-based semiconductor device

ABSTRACT

Disclosed are a group-III nitride-based semiconductor device that is grown over the surface of a composite intermediate layers consisting of a thin amorphous silicon film or any stress-relief film or a combination of them and at least one multi-layered buffer on silicon substrate, and a method of fabricating the same device. The intermediate layers that suppress the occurrence of crystal defects and propagation of misfit dislocations induced by the lattice mismatch between the epitaxial layer and substrate, ca n be grown on a part or the entirety of the surface of a silicon (001) or (111) substrate which can be single crystal or coated with a thin amorphous silicon film. Then at least one layer or multiple layers of high quality group-III nitride-based semiconductors are grown over the composite intermediate layers.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SG99/00091 which has an Internationalfiling date of Sep. 3, 1999, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating group-IIInitride-based compound semiconductor devices grown on a substrateconsisting of, for example, silicon and, more particularly, to a methodfor growing epitaxial layers of group-III nitride-based compoundsemiconductors by means of metalorganic chemical vapor deposition (to bereferred to as MOCVD hereinafter).

2. Description of the Related Art

To realize high-efficiency, high-brightness blue and ultravioletlight-emitting diodes and lasers, group-III nitride and related compoundsemiconductors have been researched and developed in recent years. As amethod for growing group-III nitride and related compoundsemiconductors, MOCVD is currently widely used.

In a typical MOCVD process, group-III nitride is grownhetero-epitaxially on a sapphire substrate which is most frequently usedat present. However, since sapphire is an insulating material andextremely rigid, it is not easy to fabricate a group-III nitride-basedsemiconductor device on a sapphire substrate. Silicon is one of theproposed substrate materials to overcome this shortcoming because of itshigh quality, large size, low cost, and the potential application tointegrated opto-electronic devices. However, due to the largedifferences in lattice constant and thermal expansion coefficientbetween the group-III nitride and silicon, it is really difficult togrow high quality epitaxial layer of group-III nitride-based compoundsemiconductor on a silicon substrate. In order to solve this problem,many attempts have been made to grow group-III nitrides on siliconsubstrates in the past decade using various kinds of materials as theintermediate layer between group-III nitride and silicon substrate.These include AIN (U.S. Pat. Nos. 5,239,188 and 5,389,571, and Appl.Phys. Lett. Vol. 72, 1998, pp. 415-417, and 551-553), carbonized silicon(Appl. Phys. Lett. Vol. 69, 1996, pp. 2264-2266), nitridized GaAs (Appl.Phys. Lett. Vol. 69, 1996, pp. 3566-3568), oxidized AlAs (Appl. Phys.Lett. Vol. 71, 1997, pp. 3569-3571), and γ-Al₂O₃ (Appl. Phys. Lett. Vol.72, 1998, pp. 109-111). In particular, by using AIN as the intermediatelayer and the molecular beam epitaxy technology, ultraviolet and violetlight-emitting diodes of group-III nitride grown on silicon substratehave been demonstrated recently (Appl. Phys. Lett. Vol. 72, 1999, pp.415-417). However, the turn-on voltages as well as the brightness ofthese diodes do not approach the performance levels of correspondingdevices grown on sapphire substrates by MOCVD. Therefore, the crystalgrowth method needs to be further improved in order to enhance thecrystallinity of the group-III nitride-based compound semiconductors andto fabricate good quality opto-electronic devices.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation and as one of its objectives, provides for a group-IIInitride-based compound semiconductor-based device which emits anddetects light with a wavelength covering from green to ultravioletranges, and is formed on a silicon substrate, having the above mentionedadvantages, e.g. high crystal quality, large wafer size, low cost,well-established processing technology, and potential application tointegrating optical devices with electronic devices on the same siliconchip.

It is another objective of the present invention to provide a crystalgrowth method for a group-III nitride-based compound semiconductor on asilicon substrate, yielding high quality p- or n-type conductor layerswith excellent characteristics so as to allow formation of an excellentp-n junction for fabrication of a group-III nitride-based light-emittingdevice, laser diode, photodetector, field effect transistor, and otheropto-electronic devices.

According to the present invention, there is provided a crystal growthmethod for group-III nitride and related compound semiconductors onsilicon substrates, comprising of the following steps:

Thermal treating a silicon (001) or (111) substrate which is a singlecrystal or coated with a thin amorphous silicon film or anystress-relief film or a combination of them in a MOCVD reactor chamberunder hydrogen ambient at a high temperature (preferably over 900° C.)for at least 5 minutes;

MOCVD-growing an ultra-thin (preferably less than 500 nm) amorphoussilicon film on a part of or the entire surface of the above mentionedsilicon (001) or (111) substrate at a lower temperature (preferablybetween 400-710° C.) using hydrogen-diluted silane as precursor;

MOCVD-growing at least one periodic or non-periodic multi-layered bufferon the top of the formed ultra-thin amorphous silicon film at a lowtemperature (preferably between 400-750° C.). Within the multi-layeredbuffer, the layers alternate between two types of compoundsemiconductors different from each other in lattice constant, energyband gap, layer thickness, and composition;

MOCVD-growing a single layer or multiple layers of group-IIInitride-based compound semiconductors over the composite intermediatelayers consisting of an ultra-thin amorphous silicon film or anystress-relief film or a combination of them and a multi-layered bufferat a higher temperature (preferably in the range of 750-900° C.); and

MOCVD-growing at least one layer or multiple layers of group-IIInitride-based compound semiconductors on the top of all of theintermediate layers to form an opto-electronic or electronic device at ahigh temperature (preferably higher than 900° C.).

According to the present invention, the group-III nitride-based compoundsemiconductor layers can be doped n- or p-type as it is MOCVD-grown overthe obtained composite intermediate layers on a silicon substrate withexcellent characteristics so as to form an excellent p-n junction forfabricating group-III nitride-based opto-electronic devices.

Additional objectives and advantages of the invention will be set forthin the description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Theobjectives and advantages of the invention may be realized and obtainedby means of the techniques and combinations thereof particularly pointedout in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated and constitute a partof the specification, illustrate presently preferred embodiments of theinvention, and together with the general description given above and thedetailed description of the preferred embodiments given below, serve toexplain the principles of the invention.

FIG. 1 is a schematic sectional view showing p-GaN and n-GaN crystalsgrown over a conventional AIN intermediate layer on a silicon substrate.

FIG. 2 is a schematic sectional view showing a GaN-based semiconductorgrown over composite intermediate layers consisting of an ultra-thinamorphous silicon film or any stress-relief film or a combination ofthem and a periodic and alternating GaN/Al_(x)Ga_(1−x)N (0<×<1)multi-layered buffer on a silicon substrate according to the preferredembodiment of the present invention.

FIG. 3 is a schematic sectional view showing an undoped GaN crystalgrown over composite intermediate layers consisting of an ultra-thin(less than 500 nm) amorphous silicon film and a three periodGaN/Al_(x)Ga_(1−x)N (x=0.2) multi-layered buffer on a silicon (001)substrate according to Example 1 of the present invention.

FIG. 4 is a graph showing the room temperature photoluninescence (PL)spectra of an undoped GaN film grown over composite intermediate layersconsisting of an ultra-thin amorphous silicon film and a three periodGaN/Al_(x)Ga_(1−x)N (x=0.2) multi-layered buffer according to Example 1of the present invention. The fill width at half maximum of theband-edge emission peak around 3.4 eV is 40 meV which is nearly 38%narrower than the narrowest value reported so far 65 meV, indicatingthat the crystal quality of GaN-based semiconductor grown on a siliconsubstrate can be significantly improved by using the compositeintermediate layers.

FIG. 5 is a graph showing x-ray diffraction profile and its rockingcurve of the (0002) reflection for an undoped GaN film grown overcomposite intermediate layers consisting of an ultra-thin amorphoussilicon film and a three period GaN/Al_(x)Ga_(1−x)N (x=0.2)multi-layered buffer according to Example 1 of the present invention.The full width at half maximum for the dominant (0002) diffraction peakat 34.6 arc-degrees is 40 arc-minutes.

This peak corresponds to the (0002) diffraction from the wurtzite GaNfilm and is much more intense than that from the silicon substrate,indicating once again that the crystal quality of GIN-basedsemiconductor grown on a silicon substrate can be significantly improvedby using the composite intermediate layers.

FIG. 6 is a schematic sectional view showing a GaN-based light-emittingdiode grown over composite intermediate layers consisting of artultra-thin (less than 500 nm) amorphous silicon film and a six periodGaN/Al_(x)Ga_(1−x)N (x=0.1) multi-layered buffer on a n-type silicon(001) substrate which is coated with a 150 nm-thick amorphous siliconfilm according to Example 2 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to a preferred embodiment of the present invention, a methodfor growing the group-III nitride-based compound semiconductors onsilicon substrates will now be described. It is to be noted that theembodiment only illustrates the invention and the invention is notlimited thereto.

As shown in FIG. 2, after standard chemical cleaning process, a silicon(001) or (111) substrate which can be a single crystal or coated on thesurface with a thin amorphous silicon film or any stress-relief film ora combination of them is thermal-treated in a MOCVD reactor chamberunder hydrogen ambient at a high temperature (preferably over 900° C.)for at least 5 minutes in order to produce a clean, oxide-free surface.

The temperature is then reduced to a low temperature (preferably between400-750° C.), and an ultra-thin preferably less than 500 nm) amorphoussilicon film is deposited on a part of or the entirety of the surface ofthe above mentioned silicon (001) or (111) substrate usinghydrogen-diluted silane as precursor in order to form a “soft” buffer onthe silicon substrate.

Subsequently, by means of our newly-developed growth technique(Singapore Patent Application No. 9801054-9, “Crystal growth method forgroup-III nitride and related compound semiconductors”), a periodic ornon-periodic multi-layered buffer in which the layers alternate betweentwo types of group-III nitride-based compound semiconductors A and Bdifferent from each other in lattice constant, energy band gap, layerthickness, and solid composition, is grown on the top of theabove-mentioned ultra-thin amorphous silicon film byatmospheric-pressure or low-pressure (60-100 Torr) MOCVD at a lowtemperature preferably between 400-750° C.). Since the growthtemperature for this multi-layered buffer is much lower than thetemperature at which a E-nitride single crystal can be formed, thebuffer layer is of an amorphous or polycrystalline state.

The growth temperature is then raised to a mediate temperature(preferably in the range of 750-900° C.), a single layer or multiplelayers of group-III nitride-based compound semiconductors is/are grownby MOCVD over the composite intermediate layers to form a secondarymulti-layered buffer and further accommodate the stress induced by thelattice mismatch between III-nitrides and a silicon substrate.

Finally the growth temperature is raised to a high temperature(preferably higher than 900° C.), then at least one layer or multiplelayers of group-III nitride-based compound semiconductors are grown overthe surface of the composite intermediate layers consisting of thepre-grown ultra-thin amorphous silicon film or any stress-relief film ora combination of them and the multi-layered buffers. According to thepresent invention, it is clear that the group-III nitride-based compoundsemiconductor layers can be doped n- or p-type as it is MOCVD-grown overthe composite intermediate layers on a silicon substrate so as to form ap-n junction for fabricating group-III nitride-based opto-electronicdevices.

It is known that as the temperature is raised from a low temperature(say 500° C.) to a high temperature (say 1,000° C.), both the ultra-thinamorphous silicon film and the amorphous or polycrystallinemulti-layered buffer will partially change to single or poly-crystallinestate due to the recrystallizing effect, which serve as seed crystalsfor the subsequent growth of the nitride-based compound semiconductorfilms. Compared with the conventional AIN single buffer shown in FIG. 1,the composite intermediate layers of the present invention demonstratethe ability to accommodate the strain arising from the lattice mismatchbetween the group-III nitride-based compound semiconductors and thesilicon substrate, and to form the seed crystal more effectively. Inother words, because the strain-accommodating and recrystallizingeffects are of crucial importance in improving the crystal quality ofthe group-III nitride-based compound semiconductors, and these effectsserve better in the composite intermediate layers of the presentinvention than in the conventional AIN single buffer, the crystalquality of the group-III nitride-based compound semiconductors will besignificantly improved by utilizing the composite intermediate layers.This is confirmed by the intense and narrow optical emission peakobserved in the PL spectra of the GaN-based semiconductor films grown byusing the composite intermediate layers of the present invention. Thex-ray diffraction data provides additional evidence to this conclusion.The detailed description will be given below in Example 1.

Note that the optimal values in total layer thickness and compositionfor the composite intermediate layers of the present inventionapparently depend on the selection of the constituent semiconductors aswell as the subsequently grown group-III nitrides and related compoundsemiconductors. At the present time since the physical origin of thecomposite intermediate layers is unfortunately not very clear, it istruly difficult to theoretically determine or predict the optimal layerthickness of the composite intermediate layers for a special materialcombination. In other words, the optimal value for a special materialcombination can now only be determined by experiment. However, theexistence of the optimal layer thickness for the composite intermediatelayers can be interpreted qualitatively as follows. Generally a bufferlayer grown at a low temperature provides seed crystals which act asnucleation sites with low orientational fluctuation to promote thelateral growth of the group-III nitrides. The composite intermediatelayers of the present invention which consist of an ultra-thin amorphoussilicon film and a multi-layered buffer provide more seed crystals aswell as additional interfaces for the misfit dislocations to terminatethan a conventional single buffer layer. However, if the compositeintermediate layers are too thin, they may neither effectivelyaccommodate the elastic strain due to the large lattice mismatch betweenthe group-III nitride crystals and the silicon substrate nor providesufficient amount of seed crystals for the subsequent growth of thegroup-III nitrides. On the other hand, if the composite intermediatelayers are too thick, they tend to bring about excessive amount of theseed crystals with high orientational fluctuation. Therefore, thereshould be an optimal layer thickness for the composite intermediatelayers.

It is also to be emphasized that the composite intermediate layers ofthe present invention, which consist of an ultra-thin amorphous film orany stress-relief film or a combination of them and at least onemulti-layered buffer can be formed not only on a silicon substrate butalso on any substrate which are presently used or may be developed inthe future, such as SiC, GaP, InP, and GaAs substrates if theconstituent semiconductors for the composite intermediate layers areselected correspondingly. The composite intermediate layers can even beformed on the surface of the epitaxial layers of group-III nitrides andrelated compound semiconductors. This characteristic implies that thecomposite intermediate layers of the present invention can be applied tothe regrowth of group-III nitride-based compound semiconductors on theas-grown nitrides.

Examples of the present invention are described below with reference tothe accompanying drawings. First, the method for growing an undoped GaNcrystal over composite intermediate layers consisting of an ultra-thin(less than 500 nm) amorphous silicon film and a three periodGaN/Al_(x)Ga_(1−x)N (x=0.2) multi-layered buffer on a silicon (001)substrate (Example 1) is described in detail. The optical properties ofthe GaN film grown over the composite intermediate layers of the presentinvention are compared with those of samples grown over conventionalintermediate layers based on the characterization results obtained by PLspectroscopy and x-ray diffraction profile. Subsequently, the layerstructure and the growth method for a specific GaN-based light emittingdiode grown over the composite intermediate layers consisting of anultra-thin (less than 500 nm) amorphous silicon film and a six periodGaN/Al_(x)Ga¹⁻N (x=0.2) multi-layered buffer on n-type silicon (001)substrate coated with a 150 nm-thick amorphous silicon film, isdescribed Example 2). These examples, however, merely exemplify themethod of practicing the technical concepts of the present invention.Therefore, the method of the present invention is not particularlylimited to the following examples in terms of, for example, the growthconditions and the combination of the materials used. Variousmodifications can be made for the growth method of the present inventionin accordance with the scope of claims.

Example 1

FIG. 3 shows a undoped GAN crystal grown over composite intermediatelayers consisting of an ultra-thin (less than 500 nm) amorphous siliconfilm and a three period GaN/Al_(x)Ga_(1−x)N (x=0.2) multi-layered bufferon a silicon (001) substrate. Referring to FIG. 3 after a chemicalcleaning process, the silicon (001) substrate is thermal-treated in aMOCVD reactor chamber under hydrogen ambient at a high temperature(preferably over 900° C.) for at least 5 minutes in order to produce aclean, oxide-free surface. The temperature is then reduced to a lowtemperature (preferably between 400-750° C.), and an ultra-thin(preferably less than 500 nm) amorphous silicon film is deposited on thesurface of the above mentioned silicon (001) substrate usinghydrogen-diluted silane as a precursor in order to form a “soft” bufferon the silicon substrate. A three period GaN/Al_(x)Ga_(1−x)N (x=0.2)multi-layered buffer is then MOCVD-grown on the top of the formedultra-thin amorphous silicon film at a low temperature (preferablybetween 400-750° C.). The film thickness of GaN and Al_(x)Ga_(1−x)N areset to 3 nm and 5 nm, respectively, corresponding to a total layerthickness of 24 nm for the multi-layered buffer. Subsequently, a 1μm-thick undoped GaN epitaxial layer is grown on the surface of thecomposite intermediate layers consisting of an ultra-thin amorphoussilicon film and a multi-layered buffer at a high temperature(preferably higher than 900° C.).

After the growth, room-temperature photoluninescence (PL) and x-raydiffraction (XRD) measurements were carried out in order to characterizethe crystalline quality of the grown undoped GaN epitaxial layer and tocompare the optical properties, more specifically, the intensity andline-width of the PL emission and XRD peaks of the undoped GaN samplegrown by using the composite intermediate layers of the presentinvention with those samples grown using the conventional methods.

According to the room-temperature PL spectrum shown in FIG. 4, the PLemission of the undoped GaN epitaxial layer grown over the compositeintermediate layers of the present invention is much more intense thanthe defects-related yellow-band emissions centered around 2.26 eV.Although it is impossible for us to directly compare our PL result withthose reported by other research groups for the GaN films grown over aconventional intermediate layer on a silicon substrate, the PL intensityof the GaN band-edge-related emission peak around 3.4 eV as seen in FIG.4 is found to be comparable to or even slightly stronger than thecorresponding value for the GaN grown in our laboratory on a sapphiresubstrate using the low-temperature-grown GaN thin film as theintermediate layer. Furthermore, the full width at half maximum of theGaN band-edge-related emission peak is 40 meV which is nearly 38%narrower than the best value achieved so far, 65 meV, recently reportedby Oshinsky et al. (Appl. Phys. Lett. Vol. 72, 1998, pp. 551-553). Thisfact indicates that the crystal quality of GaN-based semiconductor grownon a silicon substrate can be significantly improved by using thecomposite intermediate layers.

On the other hand, the XRD measurement result illustrated in FIG. 5,shows a x-ray diffraction profile and its locking curve of the (0002)refection for an undoped GaN film grown over the composite intermediatelayers consisting of an ultra-thin amorphous silicon film and a threeperiod GaN/Al_(x)Ga_(1−x)N (x=0.2) multi-layered buffer (FIG. 3).Compared with the diffraction peak from the silicon substrate at 69.3arc-degrees, the diffraction peak at 34.6 arc-degrees is dominant, andidentified as the (0002) diffraction from the wurtzite GaN crystal. Nodiffraction peak from the zinc-blende (cubic) GaN is observed, and thefull width at half maximum for the dominant (0002) diffraction peak isas narrow as 11 arc-minutes which is much better than the correspondingvalue (108 arc-minutes) reported recently by Wang et al. (Appl. Phys.Lett. Vol. 72, 1998, pp. 109-111) who used a thin γA₂O₃ film as anintermediate layer. These facts also reveal that the crystal quality ofGaN-based nitrides can be significantly improved by using the compositeintermediate layers of the present invention.

Example 2

FIG. 6 shows the schematic sectional view of a GaN-based light emittingdiode fabricated on a silicon substrate according to Example 2 of thepresent invention The detailed fabrication process is as follows.

Referring to FIG. 6, after the chemical cleaning process, the n-typesilicon (001) substrate on top of which a 150 nm-thick amorphous siliconfilm was pre-grown by chemical vapor deposition, is heated in a MOCVDreactor chamber under hydrogen ambient at a high temperature (preferablyover 900° C.) for at least 5 minutes in order to produce a clean,oxide-free surface. The temperature is then reduced to a lowertemperature (preferably between 400-750° C.), and an ultra-thin(preferably less than 500 nm) amorphous silicon film is deposited on thesurface of the above mentioned silicon (001) substrate usinghydrogen-diluted silane as a precursor in order to form a “soft” bufferon the silicon substrate. A six period GaN/Al_(x)Ga_(1−x)N (x=0.2)multi-layered buffer is then grown by MOCVD on the top of the formedultra-thin amorphous silicon film at a low temperature (preferablybetween 400-750° C.). The film thickness of GaN and Al_(x)Ga_(1−x)N areset to 3 nm and 5 nm, respectively, corresponding to a total layerthickness of 48 nm for the multi-layered buffer. Subsequently, a 0.5˜2μm-thick Si-doped GaN epitaxial layer and a 100 nm-thick Si-dopedAl_(x)Ga_(1−x)N (x=0.2) epitaxial layer are grown by MOCVD successivelyon the surface of the composite intermediate layers consisting of theultra-thin amorphous silicon film and the multi-layered buffer at a hightemperature (preferably higher than 900° C.). The temperature is thenreduced to a lower temperature (preferably between 650-850° C.) and thecarrier gas for the MOCVD growth is switched over from hydrogen tonitrogen simultaneously to grow a number of 5-15 nm-thickIn_(y)Ga_(1−y)N (0<y<0.5) epitaxial layers to form the quantum well.Subsequently, the temperature is raised to a high temperature(preferably higher than 900° C.) again and a 100 nm-thick Mg-dopedAl_(x)Ga_(1−x)N (x=0.1) epitaxial layer and a 0.5˜1 μm-thick Mg-dopedGaN epitaxial layer are MOCVD-grown successively on the surface ofIn_(y)Ga_(1−y)N quantum well. Finally, a Ni/Au contact is evaporatedonto the Mg-doped p-GaN layer and a Ti/Au contact onto the Si-dopedn-GaN layer to accomplish the fabrication of the p-n junction.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices, shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A crystal growth method for the group-IIInitride-based compound semiconductors comprising: MOCVD-growing anultra-thin layer comprising an amorphous silicon film or anystress-relief film or a combination of an amorphous silicon film and astress-relief film on a part of or the entirety of the surface of asilicon substrate at a first temperature using hydrogen-diluted silaneas a precursor; MOCVD-growing a composite intermediate layer comprisingat least one periodic or non-periodic multi-layered buffer on the top ofsaid ultra-thin layer at a higher temperature or at said firsttemperature, MOCVD-growing a single layer or multiple layers ofgroup-III nitride-based compound semiconductors over said compositeintermediate layer and said ultra-thin layer at a second temperature. 2.The crystal growth method according to claim 1, said method furthercomprising: doping a n- or p-type in said-group-III nitride basedcompound semiconductor.
 3. The crystal growth method according to claim1, wherein said silicon substrate is made of silicon wafer having anypossible orientation.
 4. The crystal growth method according to claim 1,wherein said ultra-thin layer is pre-grown on said silicon substrate. 5.The crystal growth method according to claim 1, wherein said ultra-thinlayer has a thickness of less than 500 nm.
 6. The crystal growth methodaccording to claim 1, further comprising growing group-III nitride-basedfilms in between different multi-layered buffers.
 7. The crystal growthmethod according to claim 1, wherein said first temperature is between400 to 750° C., said higher temperature is between 750 to 900° C. andsaid second temperature is greater than 900° C.
 8. The crystal growthmethod according to claim 2, wherein said ultra-thin layer is pre-grownon said silicon substrate.
 9. The crystal growth method according toclaim 3, wherein said ultra-thin layer is pre-grown on said siliconsubstrate.